The desulfurization of iron or steel melts, i.e. ferrous melts, has been of concern in the steel-making industry for some time and in the last decade or so, many systems have been described for effecting desulfurization.
Reference may be made to the following German patent publications:
22 09 902, 22 19 818, 23 12 137 and 25 27 156.
The following articles are also considered relevant:
The Bofors Desulfurizing Process, Steel Times, November 1970, pp. 805-808.
Neues Stahlentschwefelungsverfahren, Neue Hutte, 1b, Part 2, February 1971.
Influence of Injection of Solid Particles in the Ladle on Sulphide and Oxide Cleanness of Steels, Stahl und Eisen 99, pp. 1215-1217, No. 22, 5 November 1979.
The following U.S. patents, some of which are equivalent to the German patent publications mentioned, are also deemed to be relevant:
3,575,695, PA1 4,067,730, PA1 4,123,258, PA1 3,885,957, PA1 4,036,635, PA1 3,891,196, PA1 3,992,195, PA1 3,807,602, PA1 3,980,469.
It is known that desulfurization processes are of increasing effectiveness as the oxygen concentration in the melt is reduced. For this reason an iron melt is generally deoxidized before desulfurization and care is taken so that the slag which overlies the surface of the melt has a chemical composition such that there is minimum or reduced danger of reoxidation during the subsequent desulfurization.
Various processes have been developed with precisely these considerations in mind. For example, in the above mentioned German patent publication No. 22 09 902, desulfurization of the melt is carried out in a ladle lined with fire clay, magnesite, dolomite or a material with a high alumina content.
The melt is initially deoxidized, covered with a slag of predetermined chemical composition and then treated with a mixture of finely divided calcium or calcium compounds with a neutral carrier gas.
This mixture is introduced into the melt by an immersion lance opening below the surface of the melt and whereby the desulfurizing agents are blown in the latter.
The desulfurizing agents are primarily elemental calcium or calcium compounds such as CaO, CaF.sub.2, CaSi and CaC.sub.2. The carrier gas is a neutral gas such as argon.
The slag should have less than 2% iron oxide. The mouth of the lance should be more than about 2000 mm. below the surface of the iron melt and the prior deoxidation should be effected, in accordance with this reference, so that more than 0.015% aluminum remains in the metal phase.
Unless otherwise indicated herein, all percents are by weight.
These relationships imply others based upon the usual physical parameters. For example, the immersion depth of the lance should be at least 2 meters. In the specific example, a depth of 2.6 meters was used for a ladle having a height of 4 meters. Calcium has, at 1600.degree. C., a vapor pressure of 2.13 atmospheres and thus will be in a liquid state in the melt as long as it is introduced at least 1.7 m below the surface. The liquid calcium droplets appear to rise more slowly than vapor bubbles of calcium would, thereby ensuring an increased reaction time and effective utilization of the calcium blown into the melt.
While this process has been found to be effective for the latter purpose, it nevertheless has certain disadvantages including the need for a synthetic slag which must be compounded at considerable cost and effort. It also requires constant monitoring of the slag composition.